Preparation of microsphere chromiaalumina catalyst



Unite PREPAaR-ATION OF'MIGROSPHERE. GHROMIA- :ALUMINA GA'EAIEYST Thepres'entinvention relates to a process for the manufacture. of. a chromia-aluminawcatalystuinnthe .form

of trnicrospherical particles.

Catalysts which are composed predominantly mt" eoprecipitated mixtures of alumina and ehromiaii. e.,=(a1uminum oxide and chromium oxide .have. been widely iu sed 'in variousxtypes w of i hydrocarbon conversion sand reforming reactions. .The'icatalysts:oflithistype'i'are conventionaliy prepar'ed by coeprecipitatingr precursors .1 of ithe two ingredients as. arhydrogelfol'owedz by the? washing andi drying of the 'hydro'gel precipitate. The: catalysts mayi alsoTcontain 'minorsamounts ofz'one or ri'more' additi'onal promoting: oxides, sli'chl as the 'oxidesof'tplatinum, i palladium:antimony, and 1 otherr metals. s A21further description nof varioustypes of 1 catalysts -which-carez comap'os'ed pre'd'ominantly= of chromia and valuminai :mayr be found in pri'Or Patent -No. 2,236,514rrassignedzztosour assignee, and also in copending applicationziSerialiNo. I 3 13',-786," fi1ed c'tober 8y l952 in which one of theipresent applicants is a joint iinvent'or.

The coprecipitated chrornia-alumina. catalysts; such as "-t li"ose-- described hereinabovej'have "sheen tsuggested: for 'u'se b'oth in fixedbed 'and fluidized .catalyticrlreforming -processes. In -the' fixed' bed processpthe'weight zanclisize of the individual particlesv of catalyst 'areta-notsscritical l and "the preparation of catalysts for. .:this.* purposezz'has at presented a'ny unusual4tlifficultiss. 1However,::";-ini a fluidized process;' the weight andtsize offtthe catalystaparticles 'a're critical 'factors"which' greatlyminfluencmthe iticalities arise 'from thefa'ct that in a"fluidized,':process the catalyst particles must beiof-rsuch size and weight that they will remain in suspensioniin a streamzofgas.

lfthas previously been found that in afluidized process i excellent results from the I standpoint of. fluidizationz tare a'chieved--when the catalyst-particles areuspherical :and Il it has been proposed that such: spherical catalysts-,ncomn'itinly called microsphere catalysts, be made by"; spray dr'ying an aqueous slurry 'or tsolution' of. catalyst.

"When we i attempted to prepare rmicrospherical. particles of chromia-alumina scatalysts, we experienced 's'everah d iflicu'lties in obtaining zparticles. :of' satisfactory weight'*and'size. Theseidiificulties: were found to; stem "from' the -fact that the' 'maximum solids content-of a -P'urii'15able, aqueous slurry oftchromia-alurnina hydrogel is about 4 to 6%by weight. :"Wefound thatby spray drying a slurry of dhis solidscontentwwe could :notproduce a catalyst of sufficiently large particle'sizeitoi be satisfactory for fluidized reforming processes.

In accordance with our-invention we' 'have found surprisingly that the incorporation ofla' small amount of acetic acidinto a slurry of chromia-alumina-:hydrogel has the effect of decreasing the viscosity of-:the.slu rry th'ereby making it possible to prepare a pumpable-.-.slurry having a solids content as high as about by weight.

Accordingly, our 'process,which is based on the aforesaid discovery, comprises adding acetieacid toianaqueous "chromia-aluminahydrogel in an amount'to form atpump- States Patent ableslurry of "said hydrogel having a solids content from about 8 to about 15%,.and then spraydryingthe resultant slurry to produce microspheres of chromiaaluminarcatalyst. t: Byrthe. spray :drying of a slurry con- 5 ntaining' the specified quantity bfrsolids, 'it- :is possible to produce a :catalyst in--sphericat form" invwhich'the aparti'cle .size. and weight are: satisfactory :forr fluidized'reforming I processes.

t Baillie itrisvsuggested that::aceticuacidxbeeadded: to. a

silicic acid solution forrthe purpose ofz-increasing the viscosity of the solution preparatoryiozspray; drying. However, in'ourtprocess, which'involves azslurryvofihydrogel,

. the acetic: acid" vhasa theirdirectlyz opposite: effect on 1 :vis- .-:cosity.

: Our :-process can: tbewap'plied ieifectively; in 31.116 apreparafiiOllIDf .zany nrcatalyst which sis: icomposed predominantly .aofxchromiaxandtalumina. This includes catalysts which .arer. promotedrwith'minor tzamounts, 'zgenerally not'rmore than about: *bywweight," of "one:aorrmorevpromoting oxides such as. thesoxides: of: platinum,:. palladium, '=anti-.

mony, cadmium, thorium, uranium andxotherrmetals.

The "relative proportion? of :chromia and:v :alumina in ::the catalyst. does not; appear to: be significant since ,the acetic :acidphas. the. ability-to:lower the viscosity of-rthe hydrogel slurry :regardless ofi the .chromia-aluminazratio.

.Generally, however; "most :chromia-aluminacatalyst con- .itain'; from" 10 .to about -molt percent of chromia.

.The vaprocess rmay 1 be tillustrated by a typical prepara- ,-.-.tion*'as follows:

=. Solutions; of chromium acetate: and g sodiumaluminate are mixedtogethen in :accordance, with conventional practice to form a precipitate of chromia-alurnina hydrog'el, whichnisiiiallowed:tozrage for a period OfzfilIlG. Theprecipitate isxthen. rfilteredriandz-theztfilter' cake washed with rwater. Normally," atuthisnpoint the -washed filter; cake contains about '8:f12% r by weightvof solids and inorder to produce a pumpable. .slurry, it is necessary -.to .add 40 water to 'the filterucake. J'iIf no acetic'acid1 is added, .it

isnotpossiblento prepare arpumpable slurry-having a 1 solids content "greater'than: .about. 6% by weight. :How-

.even; if azsmallzamount-bfa acetic; acidis added:.thereto,

the'viscosity ofthe: cakeis lowered to such anextent that 1it is POSSiblGtOi-PICPHI'G anpumpable slurry with a solids zoontent ashigh tas'.l5%z by. weight.

:Ordinar'ily; 'we recommend that v subsequent -,or prior :tortheaddition ofacetic acid: to the filter cake, the solids .:content bewadjusted; if necessary, to .that'which is de- ::sired' for: spray drying to obtain-particles Y of optimum .size. Inthis connection, wehave found that. pumpable ;.slurries 'of the 'maximum content .ofsolidswpossible with- "out 'the addition-ofaeetic acid, i. e.; 4to 6%. by weight, will', when spray 'dried,.-produce; particles-of, too-small a size to-zbe used. conveniently in a fluidized reforming ;:process.. However, when the solids'content of the slurry ris higher, microspherical particles of larger size result fromxthe spray dryingoperation.

: Generally, for microsphericaltype catalysts to :be used .in fluidized processes, weight averagewparticle :sizes ;ranging i from 80 to about. 80-. microns in diameter. are :preferred. ..In ordento obtain particles. approximating this size, we recommendithattheaslurry*to bespray dried 65 .containafromtabout 8zto about .15% by weight-of solids,

preferably. from about 8tto'about 10% by weight.

' .The'i'amount of aaceticzacidto be .added to-theslurry idoes :notuappearctobe:critical and can be fairly. widely varied. '2' Generally, enough acidtisaused tomlowerthe 70- viscositynzof the-islurry' sufliciently to permitthe, preparaftionasof; a pumpablegslurrynofitthe desired solids content.

Anzamountawithin thexangeaof about 0.3 .to.:about- 2% by weight of the slurry, preferable from about 0.5 to 1% by weight, is ordinarily suificient for this purpose.

In order to illustrate the invention, the following example is presented. Percentages are by weight unless otherwise specified.

Example 1 A chromium acetate solution was prepared by dissolving 38.8 grams Na Cr O .2H O in 130 ml. of water to which was added 43.3 grams of 96% sulfuric acid diluted with 130 ml. of water. Forty-five (45) ml. of 95% ethanol, diluted with 130 ml. of water were added from a dropping funnel while stirring, with the temperature of the mixture being held at 150 F. or slightly lower. The mixture was stirred for 0.5 hour after the addition of the ethanol and then heated on a steam bath for /2 hour. The resulting solution was then allowed to cool under vacuum to remove the unreacted alcohol and volatile oxidation products of the alcohol. There was next added a solution of 35.6 grams of calcium acetate monohydrate dissolved in 560 ml. of water and the mixture was then allowed to stand for 12 hours and then filtered. The filter cake was washed twice with distilled water and the wash water was added to the filtrate which was then diluted to 10 liters in preparation for the subsequent precipitation step.

A sodium aluminate solution was prepared by mixing in the dry state 192 grams of heavy alumina hydrate and 120 grams of sodium hydroxide in a nickel crucible. About 200 to 300 ml. of water was added to the dry mixture with stirring and the resultant mixture was then boiled until a clear solution was obtained. The solution was diluted with water to a volume of about 500 to 600 ml. by pouring over cracked ice. It was then filtered to remove insoluble material. Just prior to the subsequent precipitation step the solution was diluted to 1 liter.

In preparation for the precipitation step, aliquot portions of the chromium acetate and sodium aluminate solutions were mixed, the pH determined, and sufficient glacial acetic acid was added to reduce the pH to 10. The calculated equivalent volume of acetic acid was then added to the bulk of the chromium acetate solution. In effecting the precipitation, the respective solu- L pH of the mixture was measured continuously and mainr tained at 10:05 by small variations in the feed rate of the chromium acetate solution. During the mixing of the solutions a precipitate formed in the reaction vessel.

The precipitate was allowed to age in the vessel for a period of about 12 hours and was then filtered using a plate and frame filter press. The filter cake was washed in place with 30 liters of tap water and then with 10 liters of distilled water. It contained about 10 mol percent of chromia and 90 mol percent of alumina.

The wet filter cake was removed from the press and found to have a solids content of about 12%. A portion of the filter cake was next prepared for spray drying by adding about 1% of glacial acetic acid by weight of the entire cake and then passing the cake through an Eppenbach colloid mill followed by hand stirring with a wooden paddle to obtain a homogeneous slurry. The slurry thus prepared was of a pumpable viscosity.

To another portion of the filter cake there was added an amount of water to reduce the solids content to about 6% by weight and a homogeneous slurry was prepared by one passage through an Eppenbach colloid mill and hand stirring. This slurry was pumpable but barely so.

It was attempted to prepare a slurry from another portion of the wet filter cake to which no water or glacial acetic acid was added. A passage through the colloid 4 mill and subsequent hand stirring failed to produce a pumpable slurry.

The slurries which have been prepared from the portions of the filter cake to which glacial acetic acid and water had been added were spray dried in a conventional disc-type spray dryer operated in the conventional manner. Microspherical particles of chromia-alumina catalyst were obtained from both slurries. The weight average particle size and particle size distribution of the spheres was then determined by microscopic examination (Handbook of Chemical Microscopy, volume I, Chamot and Mason). Also, the percent volatile material at 1000 F. and the bulk density of the catalyst were determined. The results of these determinations on the two catalysts are tabulated below:

From the data, it is seen that the slurry prepared according to the process of the invention produced considerably larger particles of catalyst when spray dried than did the slurry prepared without the addition of acetic acid. Furthermore, the catalyst produced by the process of the invention had a weight average particle size and bulk density satisfactory for fluidized reforming processes.

It is intended to cover all changes and modifications in the example of the invention, herein given for purposes of disclosure, which do not constitute departure from the spirit and scope of the appended claims.

We claim:

1. A process for the preparation of a microspherical chromia-alumina catalyst comprising 10 to 30 mol percent chromia, which comprises precipitating an aqueous chromia-alumina hydrogel at a pH of about 10, filtering to form a filter cake having 8 to 12% solids, which filter cake is not pumpable unless diluted with water to solid contents of not more than 6%, adding an amount of acetic acid within the range 0.3 to 2% by weight of said filter cake to said chromia-alumina hydrogel filter cake and adjusting the solids content, if necessary, to form a pumpable slurry having a solids content of about 8 to 15%, and spray-drying said pumpable slurry to produce microspheres of chromia-alumina catalyst having a diameter of at least 30 microns and larger than can be obtained without the acetic acid addition.

2. A process for the preparation of a microspherical chromia-alumina catalyst comprising 10 mol percent chromia, which comprises precipitating an aqueous chromia-alumina hydrogel at a pH of about 10, filtering to form a filter cake having about 12% solids, which filter cake is not pumpable unless diluted with water to solid contents of not more than 6%, and when spraydried gives microspheres having an average diameter of about 15 microns, adding about 1% acetic acid by weight of said filter cake to said chromia-alumina hydrogel filter cake to form a pumpable slurry having a solids content of about 12% and spray-drying said acetic acid treated pumpable slurry to produce microspheres of chromiay alumina catalyst having an average diameter of about 33 microns.

References Cited in the file of this patent UNITED STATES PATENTS 2,457,970 Bailie Jan. 4, 1949 2,635,082 Smith Apr. 14, 1953 2,645,619 Hoekstra July 14, 1953 FOREIGN PATENTS 586,945 Great Britain Apr. 8, 1947 644,322 Great Britain Oct. 11, 1950 

1. A PROCESS FOR THE PREPARATION OF A MICROSPHERICAL CHROMIA-ALUMINA CATALYST COMPRISING 10 TO 30 MOL PERCENT CHROMIA, WHICH COMPRISES PRECIPITATING AQUEOUS CHROMIA-ALUMINA HYDROGEN AT A PH OF ABOUT 10, FILTERING TO FORM A FILTER CAKE HAVING 8 TO 12% SOLIDS, WHICH FILTER CAKE IS MOT PUMPABL UNLESS DILUTED WITH WATER TO SOLID CONTENTS OF NOT MOTE THAN 6%, ADDING AN AMOUNT OF ACETIC ACID WITHIN THE RANGE 0.3 TO 2% BY WEIGHT OF SAID FILTER CAKE TO SAID CHROMI-ALUMINA HYDROGEL FILTER CAKE CAND ADJUSTING THE SOLIDS CONTENT, IF NECESSARY, TO FORM A PUMPABLE SLURRY HAVING A SOLIDS CONTENT OF ABOUT 8 TO 15%, AND SPRAY-DRYING SAID PUMPABLE SLURRY TO PRODUCE MICROSPHERES OF CHROMIA-ALUMINA CATALYST HAVING A DIAMETER OF AT LEAST 30 MICRONS AND LARGER THAN CAN BE OBTAINED WITHOUT THE ACETIC ACID ADDITION. 